Abstract
The unique avian vocal organ, the syrinx, is located at the caudal end of the trachea. Although a larynx is also present at the opposite end, birds phonate only with the syrinx. Why only birds evolved a novel sound source at this location remains unknown, and hypotheses about its origin are largely untested. Here, we test the hypothesis that the syrinx constitutes a biomechanical advantage for sound production over the larynx with combined theoretical and experimental approaches. We investigated whether the position of a sound source within the respiratory tract affects acoustic features of the vocal output, including fundamental frequency and efficiency of conversion from aerodynamic energy to sound. Theoretical data and measurements in three bird species suggest that sound frequency is influenced by the interaction between sound source and vocal tract. A physical model and a computational simulation also indicate that a sound source in a syringeal position produces sound with greater efficiency. Interestingly, the interactions between sound source and vocal tract differed between species, suggesting that the syringeal sound source is optimized for its position in the respiratory tract. These results provide compelling evidence that strong selective pressures for high vocal efficiency may have been a major driving force in the evolution of the syrinx. The longer trachea of birds compared to other tetrapods made them likely predisposed for the evolution of a syrinx. A long vocal tract downstream from the sound source improves efficiency by facilitating the tuning between fundamental frequency and the first vocal tract resonance.
Highlights
Evolutionary novelty in physiological and morphological features can often be traced to specific adaptations that allow organisms to exploit the fitness landscape successfully
We find that sound is produced with greater efficiency by a sound source in syrinx position and that favorable interactions between sound source and vocal tract occur with syringeal position
We investigated the interdependency between sound source position, glottal efficiency, and vocal tract length by computational simulation
Summary
Evolutionary novelty in physiological and morphological features can often be traced to specific adaptations that allow organisms to exploit the fitness landscape successfully. The avian clade is characterized by a number of striking synapomorphies, which frequently have been linked to the evolution of active flight [1]. Many of these avian features did not arise in the context of flight, and the selective regimes that led to their evolution are often poorly understood. Birds possess a laryngeal valve, but as a sound source, they have evolved a novel structure, the syrinx [5,6,7]. The archosaurian shift from producing sound with an organ located at the cranial end of the trachea to a novel structure near the tracheobronchial juncture must have conferred a selective advantage. The nature of the selective forces leading to the formation of a syrinx is still completely unknown
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